Hydraulic motors are conventionally used in wind power stations to yaw the turbine nacelle to the required angle against the wind. Once the required position is reached the nacelle is locked with powerful brakes. The turbine is kept in this position until the wind direction has changed by a certain minimum angle, whereupon the brakes are released and the turbine is rotated to a new position where it is again locked etc. The system has a number of disadvantages and limitations; the turbines do not provide optimum efficiency and the stresses on the brakes are very large which often results in slipping and damage. Another problem of wind turbines is that the vibrations occurring in the nacelle as a result of the forces acting on the rotor are not equally large over the entire sweep area of the rotor. For example, the wind speed is often higher at greater heights than nearer the ground.
These problems are well known and many systems have been developed to prevent these fluctuations being transmitted as pulsating moments to the power station structure, more particularly the tower. For example, EP-A1-0 110 807 describes a system in which a controllable flow valve is arranged in a by-pass line to the hydraulic motor which is intended to yaw the nacelle towards the wind, while accumulators are provided to reduce the pressure vibrations in the hydraulic system caused by the nacelle fluctuations about the yawing axis. The inventors of the now proposed system have tested systems of this type and have found that in order to achieve an intended effect by the by-pass valve, its throttle must be of such a large diameter that it would need a very large flow from the oil pump to get the turbine to yaw at all. This solution is therefore considered to be impossible to use in practice.
There also remains the problem that the energy which is produced by the nacelle fluctuations is not in itself transmitted to the tower but is retained in the hydraulic system through recharging the accumulators. In addition the energy stored in the accumulators acts in the wrong direction, so to speak, as when the nacelle has swung round to an end position and the accumulators have been fully recharged, the pressure is exerted in the same direction as the returning vibrations and so does not damp the vibration but increases it instead. The unwanted energy from the fluctuations is therefore not removed to any great extent.